Identify the requirement for iodoform reaction

Iodoform, $\mathrm{CHI_3}$, is formed by compounds that contain either:

• a methyl ketone group: $\mathrm{RCOCH_3}$, or
• a secondary alcohol of the type: $\mathrm{CH_3CH(OH)R}$,

because such alcohols are oxidized to methyl ketones under the reaction conditions.

Check each option

Option A: Ethyl methyl ketone

Ethyl methyl ketone is: $\mathrm{CH_3COCH_2CH_3}$ This has the methyl ketone form $\mathrm{RCOCH_3}$.

Therefore, it gives iodoform test.

Option B: Isopropyl alcohol

Isopropyl alcohol is: $\mathrm{CH_3CH(OH)CH_3}$ This is a secondary alcohol of the form $\mathrm{CH_3CH(OH)R}$.

On oxidation, it gives acetone: $\mathrm{CH_3COCH_3}$ which is a methyl ketone.

Therefore, it gives iodoform test.

Option C: 3-Methyl-2-butanone

Structure: $\mathrm{CH_3COCH(CH_3)CH_3}$ This also contains the methyl ketone group $\mathrm{RCOCH_3}$.

Therefore, it gives iodoform test.

Option D: Isobutyl alcohol

Isobutyl alcohol is: $\mathrm{(CH_3)_2CHCH_2OH}$ This is a primary alcohol, not of the form $\mathrm{CH_3CH(OH)R}$.

On oxidation, it forms an aldehyde/acid, not a methyl ketone.

Therefore, it does not give iodoform test.

Conclusion

The compound from which iodoform cannot be prepared is: $\boxed{\text{D: Isobutyl alcohol}}$